Monday, June 8th, 2015

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Pacific Northwest Earthquakes: The REAL Big One

by Chris Thompson

The first day of my Earthquake Engineering course at UC Berkeley, Professor V.V. Bertero, the father of earthquake engineering, opened with the following –

Earthquakes do not kill people. Structures that fail in earthquakes kill people.

Structural Engineers design building structures to resist loading from a variety of environmental and manmade sources. One source of loading that we must design building structures for is earthquakes.

Our understanding of the intensity of seismic loading in Oregon has evolved significantly in recent times. If you look around at the breathtaking landscape of our state, the evidence of an uneasy earth below us is widespread. The dormant Cascades, from Mt. Ashland to Mt. Hood are prime examples of past upheavals of the earth. The coastal swamps and submerged forests yield evidence of cataclysmic movements of the tectonic plates and the subsequent tsunamis.

We know that extreme earthquakes have occurred in the Pacific Northwest in the past – in fact, scientists have pinned down the date of the last earthquake of this type in our region. Through the clever combination of carbon dating sands washed ashore from tsunamis and historical records of those tsunamis striking coastal Japan, the date of the last “mega-earthquake” off of the coast has been estimated to be January 26, 1700. The magnitude of this earthquake is estimated to be roughly 9.0 on the Richter Scale.

We have not experienced an earthquake of such magnitude in modern history in the United States. The 1989 Loma Prieta (M 6.9), 1994 Northridge (M 6.7), and 2001 Nisqually (M 6.8) are our most recent reminders of the destructive power of earthquakes on the west coast. These earthquakes unfortunately contain far less destructive power than the predicted mega earthquake of the Cascadia Subduction Zone.

One of the main differences between the above referenced earthquakes and the Cascadia event will be in the area that is affected by the shaking. Our recent experience with earthquakes in the western states has been with concentrated areas of damage near the fault rupture. The Cascadia event will rupture upwards of 300 miles of fault off of the Oregon Coast. Therefore, areas of heavy damage could extend from Northern California to Northern Washington. The area of significant structural damage and economic impact will be unprecedented in our history. The experience of Superstorm Sandy is a grim reminder of widespread destruction form nature’s fury.

The other main difference, and possibly more significant impact to the built environment will be the duration of string ground shaking. In earthquakes such as Loma Prieta, the duration of strong ground shaking – the shaking that damages vulnerable buildings – has been limited to 30 seconds or less. The 1994 Scotts Mills earthquake in Oregon had roughly 5 seconds of strong ground shaking. The predicted duration of strong ground shaking in the Cascadia event is between 90 seconds and 3 minutes.

A person typically will detect strong ground shaking at about 5 seconds. Beyond that, we will know what is happening and experience what will seem like an eternity before the ground ceases to rumble. The duration of the shaking will subject buildings and other structures to more cycles of damaging motion than ever experienced in modern history. Our Building Codes do not explicitly address duration of earthquakes, however modern structures are designed to absorb and dissipate damaging motion. We just haven’t experienced what will happen in Cascadia in the great laboratory of the built environment. The real danger lies within the great stock of structures that were not designed to absorb the energy of earthquakes.

 

Graphics Credit: http://www.ecy.wa.gov/programs/sea/coast/images/subduction_9.gif

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